Investigation on bainite transformation and metastable omega/orthorhombic martensite phases in as-quenched Cu-bearing ferrite steel

Abstract

The as-quenched microstructures of low-carbon Cu-bearing ferrite steel are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the granular bainite formation in Cu-bearing ferrite steel is attributed to a mixed mode of diffusional and displacive mechanisms. Besides, the particle-like or plate-shaped ω phases, along with thin-plate martensites consist of twin, are observed in the Mn1.4Cu-bearing steel, and the orientation relationship is determined as: [2 −1 −10]ω // [0 1 1]α and (0 1 −1 0)ω // (2 −1 1)α. The ultra-fine or ellipsoidal shaped nanoscale orthorhombic martensite phases, along with laths martensites consist of a high density of dislocations, are observed in the Mn0.66Cu-bearing steel, and the orientation relationship is determined as: [0 1 −1]o//[−1 1 1]α and (−1 1 1)o//(1 0 1) α. A model is proposed to describe the overall martensite transformation kinetics by taking into account the Mn content, heterogeneity of C content and dislocation reactions play a major role in the subsequent martensite transformation.